Investigation of a Novel Five-Phase Modular Permanent-Magnet In-Wheel Motor

1. Investigation of a Novel Five-Phase Modular Permanent-Magnet In-WheelMotor Ping Zheng , Yi Sui , Jing Zhao , Chengde Tong , T.A. Lipo , Fellow, IEEE, and Aimeng Wang Department of Electrical Engineering, Harbin Institute of Technology, Harbin, 150080 China Department of Electrical and Computer Engineering, University of Wisconsin Madison, Madison, WI, 53706 USA Department of Electrical Engineering, North China Electric Power University, Baoding, 071003, China IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011, Page(s): 4084 ~ 4087 • Adviser :Ming–Shyan Wang • Student :Ming- Yi Chiou • Student ID: Ma120122

2. Outline • I. INTRODUCTION • II. THE CHOICE OF ELECTROMAGNETIC SCHEMES • III. DESIGN OF FIVE-PHASE FAULT-TOLERANT MOTOR • IV. FAULT-TOLERANT ABILITY • V. CONCLUSION

3. I. INTRODUCTION THE permanent-magnet synchronous motors (PMSMs) are more and more widely , owing to advantages of high efficiency and power density compared. traditional induction machines. However, for applications that require extremely high reliability, such as military equipments, aircrafts and electric vehicles, the traditional three-phase PMSMs are confronted with severe challenge, so some researchers turn to the investigation of multiphase PMSM systems for better fault tolerant capability. Generally, the torque of traditional three-phase PMSM drive system will decreases dramatically in case of major failures, especially for in-wheel motor driven vehicles . multiphase motors are valuable , they are reliable and can perform well with relatively low technology support . Compared with traditional three-phase PMSMs, the multiphase fault-tolerant PMSM could greatly increase the reliability and fault-tolerant capability of the electric vehicle drive system . In this paper, analysis and finite- element method (FEM) simulations are made to a novel five-phase modular permanent-magnet in-wheel motor.

4. II. THE CHOICE OF ELECTROMAGNETIC SCHEMES Compared with traditional three-phase PMSMs, multiphase PMSMs have some unique characteristics, such as complete electric isolation, effective magnetic isolation, complete physical isolation between phases, effective thermal isolation, the ability to limit short-circuit current between phases, and the ability to tolerate short circuit between turns. So in order to design a multiphase PMSM with fault-tolerant ability, all of the previously mentioned characteristics must be considered.

5. The Choice of Phase Number: • The multiphase PMSM should continue to produce the rated power in case of single phase failure. Hence, if the phase number of the machine is N, the power of each phase should be designed to be 1/(N-1) times of the rated power. If less phase number is employed, the power rating of one single phase should be higher. For example, if the machine is four-phase PMSM, the power of each phase should be over-rated by about 33.3%, while for the traditional three-phase PMSM, it should be 50%. In order to realize electric isolation between phases, each phase is driven by a single four-switch inverter, so the number of the power components increases with the phase number. Moreover, the probability of phase failure also grows with the increase of phase number. To choose appropriate phase number, the complexity, cost and fault-tolerant ability of the system need to be considered. In this paper, the five-phase scheme is employed and the over-rated power of one phase is 25% of normal rating.

6. 2) The Choice of Slot/Pole Combination: Each phase of the motor should be an independent module. Compared with double-layer windings, the winding factor of single-layer armature winding is smaller. Moreover, the harmonic components of back electromotive force (EMF) and the magneto motive force (MMF) of single-layer windings is higher, so the torque pulsation and the noise of the motor is quite severe. Theoretically, the number of the stator slots is integer times of five. Considering unilateral magnetic force, even number of slots should be preferred. Due to the low winding factor of the armature winding, pole number should be close but not equal to the slots number. If the slots number is Q, the poles pairs is p and n is an integer larger than zero, then the combination could be calculated by (1) and (2): Considering the in-wheel drive application, the numbers of slots and poles are designed to be 40 and 42 respectively with the armature winding factor of 0.985.

7. III. DESIGN OF FIVE-PHASE FAULT-TOLERANT MOTOR A. Model Design of the Motor In this section, 40-slot concentrated winding stator is investigated. There are four coils for each phase and the electrical angle shift of adjacent phases winding is 72 . The tooth between adjacent phases is called fault-tolerant tooth. Considering actual requirement, outer-rotor structure is employed and Nd-Fe-B-45SH is selected as permanent magnet material. A novel five-phase fault-tolerant PMSM was designed here, and the main dimensions of the five-phase motor are listed in Table I. The stator slots were designed deep enough to obtain large inductance. Magnetic path of the yoke is short, so the magnetic flux density of yoke is designed to be relatively higher.

8. B. Shape of the Fault-Tolerant Tooth In this paper, only width of the fault-tolerant teeth is investigated, as shown in Table II. Results indicate that 12 mm is the optimal width of fault-tolerant teeth, so 12 mm is selected for the following analysis. Fig. 1 shows the finite element model of the in-wheel motor with 12 kW rated power and 450 rpm rated speed.

9. C ‧Electromagnetic Performance of the Motor Fig. 2 shows the no-load back EMF of the motor. The amplitude of the back EMF is 288 V, and the waveform is approximately trapezoidal. The rated torque of the motor, denoted by TN , is 254.4Nm . Fig. 3 shows the cogging torque of the motor. It can be seen that fractional slots are employed in the fault-tolerant motor to minimize the cogging torque. In Fig. 4, the motor was fed with rated current. It is observed that the average torque Tav is 260.3Nm larger than TN , Due to the increase of phase number, pulsation of the load torque is only 1.23%.

10. IV. FAULT-TOLERANT ABILITY • In this section, the following failures are considered: • Open Circuit of One Phase: • Open Circuit of Two Adjacent Phases: • Open Circuit of Two Non-Adjacent Phase: • One Phase Short Circuit:

11. Open Circuit of One Phase: Suppose a-phase is open circuit, the current of other healthy phases should be increased to 1.25 times of the normal value to provide the same torque. In Fig. 5, the average torque value is 240Nm , basically meeting the requirement of torque. In case of a-phase open-circuit, the magnetic field is asymmetric, so the pulsation of load torque is larger with the value of 28.4%. Increasing the current amplitude of other healthy phases could compensate the output torque given the loss of one phase. Current angles of healthy phases can be controlled to reduce torque pulsation. Besides, currents of healthy phases should have the same amplitude value and maintain symmetrical. some typical equations are given by(3):

12. Fig. 6 shows the torque behavior with the 36 angle shift of current vector from phase B to phase A. The average torque value is 239 , with 10.5% pulsation. The average torque is a slightly decreased, but the torque pulsation is reduced dramatically, which demonstrates that the motor can work well under condition of one phase open circuit.

13. 2) Open Circuit of Two Adjacent Phases: In this section, suppose A, B phases are open circuit. Fig. 7 shows the torque performance with three healthy phases fed with symmetric current. The amplitude of current is 1.25 times of rated value leading to reduced average torque of about 131.7 Nm with pulsation of 21.2%.

14. 3) Open Circuit of Two Non-Adjacent Phase: In this section, suppose A, C phase is open continuously due to fault. Fig. 8 shows the torque performance with three-phase fed with symmetric current. The amplitude of current is 1.25 times of the rated with the value of 20.83 A. The average torque value is 168 Nm, with 26.2% pulsation. The results indicate that the motor could survive two non-adjacent open-circuit conditions, with a little torque reduced. The results of above analysis indicate that the machine can survive with acceptable value and pulsation under open circuit fault of two phases.

15. 4) One Phase Short Circuit: In this section, suppose a-phase is in short circuit fault. In this case, currents of other healthy phases are controlled to be 1.25 times of the rated value to maintain torque output. Fig. 9 shows the current of short-circuit phase.

16. V. CONCLUSION • 1) For five-phase fault-tolerant PMSM, the slot number is recommended to be integer times of 10, and the pole number is recommended to be the slot number 2. • 2) The 40-slot/42-pole combination is selected for the wheel driven application in this paper, and the electromagnetic design shows that it can meets the requirements in EV application. • 3) The fault-tolerant tooth was optimized with FEM, and better torque curve is achieved. 4) The fault-tolerant ability was analyzed under four fault conditions; research results indicate that the drive system can survive these fault conditions. • 5) Through proper current control strategies, different fault tolerant performances can be obtained.

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